1. The Field of the Invention
The present invention relates to digital storage devices having a rotating media and more specifically to systems and methods for finitely positioning a read/write slider in such a storage device.
2. The Relevant Art
Computer systems generally utilize auxiliary storage devices onto which data can be written and from which data can be read for later use. A direct access storage device (DASD) is a common auxiliary storage device in which data is stored in known locations and accessed by reference to those locations. A hard disk drive is a type of DASD that incorporates rotating magnetic disks for storing data in magnetic form on concentric, radially spaced tracks on the disk surfaces.
In a typical hard disk drive, transducer heads driven in a path generally perpendicular to the drive axis are used to write data to and read data from addressed locations on the disks. These transducer heads are mounted on sliders that are comprised of a ceramic substrate with an air-bearing surface. Current hard disk drives also typically utilize an actuator, positioned by a voice coil motor that is connected to the slider by a support arm assembly. The voice coil motor moves the actuator arm, which then moves the slider to the desired track and maintains the selected position over the track centerline during a read or write operation.
FIG. 1a shows one example of a digital storage device of the prior art. Shown in the depicted embodiment is a direct access storage device (DASD) in the form of a magnetic hard disk drive unit 100. The disk drive unit 100 is shown illustrated in a simplified form sufficient for an understanding of the prior art and as one example of the various types of storage devices that might employ the system and methods of the present invention.
The illustrated disk drive unit 100 includes a plurality of disks 102 each having at least one magnetic storage surface 104. The disks 102 are mounted in parallel for simultaneous rotation on and by an integrated spindle and motor assembly 106. Data stored on the surface 104 of each disk 102 is read from and/or written to by a corresponding transducer head mounted on the slider 107 coupled to either an interior arm 108 or exterior arm 109. Arms 108 and 109 are movable across the disk surface 104. As shown in FIG. 1b, the arm 108 supports two head gimbal assemblies (“HGA”), each of which includes a base plate 116, a suspension arm 114, and the slider 107. Rotating the arm 108 causes the slider 107 to be moved in a path that allows it to access the different tracks on the disk surfaces 104. Contrary to the interior arms 108, only one HGA is attached to the exterior arms 109 in order to access only the outer surface 104 of the respective disk 102.
In operation, a voice coil motor 112 controls the plurality of interior arms 108 and the exterior arms 109. The arms 108 and 109 move in a synchronous, rotary direction about the pivot assembly 113 in order to position the slider 107 above data tracks located on the magnetic surfaces 104.
In modern hard disk drive systems the track-to-track spacing, or track pitch on the magnetic disks is decreasing at a dramatic rate. Consequently, currently available designs for actuator and head gimbal assemblies are experiencing increasing difficulty in adequately positioning the slider 107 with sufficient precision over the centerline of the data track during read and write operations. The problem arises from the relative amplitude of the vibration modes of the actuator arm and HGA relative to the track pitch. The vibration modes are excited by airflow over the arms and HGA from the rotation of the disks, airflow buffeting at the edges of the spinning disks, and forces from the currents in the spindle motor and VCM, in addition to other well known sources of excitation. Since the excitation sources, i.e., air flow and external sources, are independent of the track pitch, it is clear that to improve the tracking precision there is a required improvement in the design of the actuator and servo systems.
One method that has been developed to improve finer track positioning is the use of HGAs having multiple positioning means. The essence of this approach is to introduce a secondary actuator that has a lower moving mass than the traditional actuator driven by a VCM. This, in combination with an appropriate servo design, allows a higher bandwidth servo system, with the result that the frequencies associated with the above mentioned excitation sources can be rejected. In operation, generally, a motor such as a voice coil motor performs the gross positioning of the slider, and a secondary actuator is used to achieve the fine positioning. Typically, the secondary actuators are mounted in the base plate, the load beam (not shown), or the gimbal (not shown) of the HGA. The secondary actuators often include piezoelectric transducers (“PZTs”) or miniature voice coil motors as the means to provide the fine positioning movement.
HGAs having multiple positioning means are also referred to as the dual stage actuators. Such dual stage actuators are implemented in place of each HGA, so that for interior arms 108 there are two dual stage actuators attached to it and for exterior arms 109 there is only one dual stage actuator attached to it. The dual stage actuators attached to the interior arms 108 simultaneously access the disk surface 104 directly above and below the interior arm 108. The dual stage actuator attached to the exterior arm 109 accesses the disk surface 104 directly below or above the exterior arm 109 depending on whether the exterior arm is the first arm or the last arm in the disk stack assembly. One example of a dual stage actuator is described in U.S. Pat. No. 5,764,444.
An example of a dual stage actuator is shown in FIG. 2. The dual stage actuator 200 comprises a slider 107 that carries the transducer heads (not shown), a base plate 202, first and second PZTs 204 that are used as the secondary actuators, and a suspension 208. Traces 209 carry electrical signals to and from the transducer heads. The electrical signals are used to read and write data on the disk surfaces 104.
The arrows 210 and 212 show the direction of movement caused by the PZTs 204 upon the selective application of voltages to them. The PZTs 204 work in concert to finitely position the slider 107. In order to position the slider 107, one of the piezoelectric transducers 204 is configured to extend while the other contracts. This is accomplished by connecting the piezoelectric transducers 204 with opposite polarities.
For example, when a voltage is applied in such a manner that one of the piezoelectric transducers 204 extends, and the other piezoelectric transducer 204 contracts, the slider 107 moves to the left 210. When the opposite voltage is applied, the slider 107 moves to the right 212. This has proven to be a reliable method of achieving finite positioning of the slider 107. The combination of the dual stage actuators and a suitable servo feedback system provides the capability of reliably positioning the slider 107 on the center line of tracks that are closer and closer to each other and, therefore, achieving greater hard disk data densities.
One problem that arises in the current state of the art for dual stage actuators is that the moving mass of the suspension arm 208 and slider 107 can cause a reaction force or torque in the arms 108 or 109. This reaction torque induces bending stress and torsional stress in the arms 108 and 109, and can excite the vibrational modes in the arms. The amplitude and frequency of these vibration modes can limit the track following performance of the hard disk drive 100 because of the offtrack motion that they cause at the slider 107 mounted on the suspension arm 208.
Two types of vibration affect the suspension arm 208 and slider 107. The first vibration mode is the sway mode. The sway mode is also described as an “in-plane bending” and is caused by the inertial forces due to the acceleration of the actuator by the VCM torque. The in-plane bending causes the slider 107 to vibrate to either side of the data track that the slider 107 is attempting to read or write. The second type of vibration is the torsional mode, which is caused by unbalanced inertial torsional forces about the longitudinal axis (or torsional axis) passing through the center plane of the arm 109. The torsional mode causes the slider 107 to twist in clockwise and counterclockwise directions perpendicular to the suspension arm 208. This also causes the off track motion of the slider 107 preventing it from being aligned with the center of the desired track.
For a hard disk drives with multiple arm actuators, such as the hard disk drive 100 described in FIG. 1, the multiple piezoelectric transducers 204 of each dual stage actuator 200 attached to arms 108 or 109, are typically driven in the same direction in order to minimize the number of control lines 214 of FIG. 2. For interior arms 108 situated between the plurality of disks 102 of FIG. 1, the reaction force of the dual stage actuators on the torsional modes of the arm 108 are balanced due to the symmetrical manner in which two dual-stage actuators are coupled to the arm 108. Thus, the torsional vibration modes are not excited for the interior arms 108 when the associated dual stage actuators are operated in the same direction.
It is apparent that this balanced operation cannot be achieved for the exterior arms 109, since each exterior arm supports only one dual stage actuator. It can be confirmed using modeling schemes that the torsional modes of the outer arms 109 are excited by the moving mass of the single dual stage actuator. Since the mass of the suspension arm 208 and slider 107 is offset from the torsional axis of the outer arm 109 of FIG. 1, the excitation drives both sway and torsion modes. The offtrack error resulting from the torsional vibration of the exterior arms 109 limits the attainable tracking precision performance of the hard disk drive 100 since, typically, the frequencies of the sway and torsion modes are beyond the frequency range in which the servo system is effective. A similar problem exists in the single stage actuator head disk assemblies. In these assemblies, the unbalanced inertial forces of the single HGA on the outer arm excites the torsional modes of the outer arm when the head stack assembly is accelerated by the VCM.
In one current practice to address this problem, a passive or “dummy” mass is placed on the exterior anus so that the combined inertia of the dummy mass and the single HGA act to prevent the excitation of the end arm torsional mode. FIG. 3 illustrates one embodiment of a dummy mass 300 of the prior art attached to the outer arm 109. However, the application of the dummy mass is not effective in the case of a head stack assembly with dual stage actuator since the excitation of the end arm torsion mode is caused by the acceleration of the moving mass of the dual stage actuator. The moving mass consists of slider 107, suspension arm 208, and the traces 209. This is in contrast to the single stage actuator case, where the end arm torsion mode is excited by acceleration of the head stack assembly by the voice coil motor. Thus it is apparent that a new solution is required in the case of the head stack assembly with dual stage actuator.
Accordingly, it should be apparent that a need exists for an improved method to minimize or eliminate the effect of the offset moving mass associated with an exterior arm dual stage actuator such that a slider carrying the read/write transducers can be quickly and accurately positioned over the centerline of a disk drive track in response to control signals from the disk drive and positioning signals from the disk surface of the disk drive.